Crude human lymphoblastoid interferon (IF) has less ribonuclease (RNase) activity than equivalent primary leukocyte IF. RNase activity was abolished when the IF was purified by methods that differed from those that had failed to eliminate similar activity from leukocyte IF. It is thought unlikely that exogenous RNase plays a major role in the antiviral action of IF preparations. Basic ribonuclease activity, rather than simply the presence of protein could be detected using polynucleotide-polyacrylamide gel electrophoresis. Polyacrylamide gels were prepared containing low concentrations of a polynucleotide substrate. Spermine, present in high amounts during electrophoresis, enables the basic RNases to migrate in the presence of polyanionic substrates. Electrophoresis of the RNases and incubation of the gel at neutral pH followed by staining results in a colored gel containing clear bands which define regions of enzyme activity. Higher resolution of activity regions was achieved by using polyacrylamide gel electrophoresis in one direction, followed by polynucleotide-polyacrylamide gel electrophoresis in a direction perpendicular to the first migration. Enzyme activities are visualized as colorless spots on a purple slab. Preliminary results also indicate that pretreatment leukemia patients have an additional lymphocyte nuclease activity when compared to lymphocyte activity patterns obtained from normal volunteers.